(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of creating a molded package structure for flip chips using a one step mold compound injection process.
(2) Description of the Prior Art
High density interconnect technology frequently leads to the fabrication of multilayer structures in order to connect closely spaced integrated circuits with each other. A single substrate serves as an interconnect medium to which multiple chips are connected, forming a device package with high packaging density and dense chip wiring. The metal layers that make up the interconnect network and the via and contact points that establish connections between the interconnect networks are separated by layers of dielectric (such as polyimide) or insulating layers. In the design of the metal interconnects, strict rules must be adhered to in order to avoid problems of package performance and reliability. For instance, the propagation directions of primary signals must typically intersect under angles of 90 degrees to avoid electrical interference between adjacent lines. It is further required that, for considerations of photolithography and package reliability, planarity is maintained throughout the construction of multi-layer chip packages. Many of the patterned layers within an interconnect network form the base for subsequent layers, their lack of planarity can therefore have a multiplying effect of poor planarity on overlying layers.
Quad Flat Packages (QFP) have in the past been used to create surface mounted, high pin count integrated packages with various pin configurations. The electrical connections with these packages are typically established by closely spaced leads that are distributed along the four edges of the flat package. This limits the usefulness of the QFP since a high I/O count cannot be accommodated in this manner. To address this problem, the Ball Grid Array (BGA) package has been created whereby the I/O points for the package are distributed not only around the periphery of the package but over the complete bottom of the package. The BGA package can therefore support more I/O points, making this a more desirable package for high circuit density with high I/O count. The BGA contact points are solder balls that in addition facilitate the process of flow soldering of the package onto a printed circuit board. The solder balls can be mounted in an array configuration and can use 40, 50 and 60 mil spacings in a regular or staggered pattern.
Where circuit density keeps increasing and device feature size continues to be reduced, the effect of the interconnect metal. within the package becomes relatively more important to the package performance. Factors that have a negative impact on circuit performance, such as line resistance, parasitic capacitance, RC-delay constants, crosstalk and contact resistance, have a considerable impact on the package design and its limitations. A significant power drop may for instance be introduced along the power and ground buses where the reduction of the interconnect metal does not match the reduction in the size of the device features. Low resistance metals (such as copper) are therefore finding wider application in the design of dense semiconductor packages.
Increased input/output (I/O) requirements combined with increased performance requirements for high performance Integrated Circuits (IC""s) has led to the development of Flip Chip packages. Flip chip technology fabricates bumps (typically Pb/Sn solder) on A1 pads and interconnects the bumps directly to the package media, which are usually ceramic or plastic based. The flip-chip is bonded face down to the package through the shortest paths. These technologies can be applied not only to single-chip packaging, but also to higher or integrated levels of packaging, in which the packages are larger, and to more sophisticated package media that accommodate several chips to form larger functional units.
The flip-chip technique, using an area array, has the advantage of achieving the highest density of interconnection to the device combined with a very low inductance interconnection to the package. However, pre-testability, post-bonding visual inspection, and Temperature Coefficient of Expansion (TCE) matching to avoid solder bump fatigue are still challenges. In mounting several packages together, such as surface mounting a ceramic package to a plastic board, the TCE mismatch can cause a large thermal stress on the solder lead joints that can lead to joint breakage caused by solder fatigue from temperature cycling operations.
Prior Art substrate packaging uses ceramic and plastic Ball Grid Array (BGA) packaging. Ceramic substrate packaging is expensive and has proven to limit the performance of the overall package. Recent years have seen the emergence of plastic BGA packaging, this packaging has become the mainstream design and is frequently used in high volume BGA package fabrication. The plastic substrate BGA package performs satisfactorily when used for low-density flip-chip IC""s. If the number of pins emanating from the IC is high, that is in excess of 350 pins, or if the number of pins coming from the IC is less than 350 but the required overall package size is small, or if the chip power dissipation is high (in excess of 4 Watts per chip), the plastic structure becomes complicated and expensive.
It is therefore the objective of packaging ball grid array and flip-chip packages that the chip is mounted on the surface of a package substrate. The contact points of the flip-chip Integrated Circuit (IC) device make contact with contact points in the top surface of the Ball Grid Array (BGA) substrate, the substrate re-distributes (fan-out) the flip-chip IC contact points. The lower surface of the substrate has the contact point (balls) that are connected to the surrounding circuitry and that form the interface between the BGA/flip-chip IC contact balls and this surrounding circuitry. It must thereby also be understood that the original contact balls of the flip chip IC device are encased in a material (for instance epoxy) for protection of these balls. The epoxy is encased between the lower surface of the flip-chip IC device and the upper surface of the BGA substrate. This epoxy layer is referred to as underfill since it is filled in under the flip-chip device. The underfill is normally put in place using a separate process of dispensing epoxy liquid under the die followed by curing of the epoxy. IC devices that are packaged using a flip chip and that have requirements of high power dissipation normally require a heatsink that is attached to the surface of the flip chip die. Only the backside of the flip chip is exposed and is suitable for the attachment of the heatsink. Since the heatsink is only attached to the (backside) surface of the IC device, great care must be taken not to induce stress on the backside of the IC device. If too much force or stress is used during the process of attaching the heatsink to the die, the die can easily crack or break. If on the other hand a larger surface area is created that is parallel to the surface of the backside of the IC device, the stress can be significantly reduced.
FIG. 1 shows a cross section of a typical flip chip package with underfill and a heatsink. The IC 10 enters the process as a separate unit with the contact points (balls 18) attached to the bottom of the chip 10. The IC 10 is placed in a cavity 22 that is formed by the spacers 14 between the heatsink 16 and the substrate 12. While the chip 10 is contained in cavity 22, the underfill 21 under the surface of the IC chip 10 is injected or filled by capillary action. The balls 20 connected to the lower surface of the substrate 12 make contact with the surrounding circuitry. It should be noted in FIG. 1 that the sides of the underfill 21 are sloping such that the physical contact between the underfill 21 and the substrate 12 is extended beyond the dimensions of the bottom surface of the chip 10. This is a normal phenomenon with liquid underfill, which enhances heat interchange between the substrate 12 and the IC chip 10.
U.S. Pat. No. 5,898,224 (Akram) shows a Flip chip assembly with underlayer and a heatsink and an outer top encapsulant.
U.S. Pat. No. 5,726,079 (Johnson) and U.S. Pat. No, 5,883,430 (Johnson) show a flip chip encapulated on the bottom and sides, a heatsink on top and an underlayer.
U.S. Pat. No. 5,817,545 (Wang et al.) teaches a process for underfill for a flip chip.
U.S. Pat. No. 5,896,271 (Jensen et al.) recites a flex substrate with a heatsink on top.
U.S. Pat. No. 5,385,869 (Liu et al.) shows a flip chip and a mold process.
A principle objective of the invention is to create underfill for flip-chip type Integrated Circuits (IC""s) in one processing step by forcing a mold compound to flow under the die rather than over the backside of the die.
Another objective of the invention is to relieve thermal and mechanical stress between the flip-chip die and the heatsink that is attached to the die by creating a uniform flat surface as an interface between the die and the heatsink. This uniform flat surface distributes the stress that is introduced by the attachment of the heatsink over a larger area, thereby reducing the stress to which the die is subjected (during heatsink attachment).
In accordance with the objectives of the invention a new method is provided to insert the underfill for flip-chip semiconductor devices. An IC chip is provided with solder bumps. The flip-chip is entered into an enclosed space, the heatsink forms the top of the enclosed space, the substrate forms the bottom of the enclosed space. The enclosed space is filled with a mold compound. This mold compound now surrounds the IC chip thereby including the area below the IC. The step of inserting the underfill as a separate processing step has thereby been eliminated.